1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
In image forming apparatuses utilizing an electrophotographic process, such as copying machines, printers, facsimiles and the like, a method including a toner image forming step, a toner image transferring step, and a transferred image fixing step is widely used. At the toner image forming step, a toner image is formed on a surface of a photoreceptor. At the toner image transferring step, the toner image formed on the surface of the photoreceptor is transferred onto a recording medium by an effect of an electrical field, At the transferred image fixing step, a permanent fixing image is formed on the recording medium by heating and pressing the unfixed toner image and the recording medium. At the fixing step, a fixing section including a fixing roller as a fixing member and having a heating section therein, and a pressure roller as a pressure member arranged in pressure contact with the fixing roller, is used, and the toner image is fixed onto the recording medium by introducing the recording medium bearing the unfixed toner image into a pressure contact area (a fixing nip area) between the fixing roller and the pressure roller, and heating and pressing the recording medium. In the image forming apparatuses utilizing the electrophotographic process, color image forming apparatuses for forming a color image are rapidly spreading. The color image has more amount of toner attachment per unit area compared with a black and white image. Accordingly, more amount of heat is required in order to sufficiently fix a toner onto the recording medium. In a configuration where the toner image is fixed onto the recording medium using the above-described fixing apparatus, it is necessary to rapidly heat the toner and the recording medium in the fixing nip area that typically has a size of around 5 to 6 mm, and is formed between the fixing roller and the pressure roller. Therefore, for example, a surface temperature of the fixing roller is set to around from 170° C. to 180° C., which is much higher than a fusing temperature of the toner.
The color image forming apparatus as described above, also requires, likewise with respect to a monochromic image forming apparatus for forming a black and white image, speeding up of an image forming speed, reduction of its power consumption, and the like. To meet these requirements, typically, a width of the fixing nip area (a fixing nip width) is increased, and a heating temperature for fixing an unfixed toner image onto the recording medium (a fixing temperature) is lowered. However, when the fixing nip width is increased, there arises a possibility that excessive heat energy is displaced to the recording medium to cause a surface temperature of the fixing roller to decrease lower than a setting temperature when carrying out a continuous paper feeding, thus causing a fixing failure. In addition, when external diameters of the fixing roller and the pressure roller are increased in order to increase the fixing nip width, an amount of heat discharge from each roller is increased to cause a tendency to have a fixing failure. Moreover, the fixing nip width can be increased also by increasing a thickness of an elastic layer containing rubber and the like on the surfaces of the fixing roller and the pressure roller. However, heat conductance from a heat source inside the fixing roller to the surface of the fixing roller is decreased, still causing a tendency to have a fixing failure. Therefore, it is difficult to stably form a color image fixed with high fixing level onto the recording medium, while achieving speeding up of an image forming speed and reduction of its power consumption, only by increasing the fixing nip width. As described above, a fixing method in which heating and pressing of the toner image and the recording medium are carried out at the same time only in the fixing nip area, requires a large amount of power consumption in order to fix the toner image onto the recording medium.
Furthermore, there is known a concurrently transferring and fixing method in which using a transferring and fixing section including a transferring and fixing roller having a heating section therein, and a pressure roller arranged in pressure contact with the transferring and fixing roller, a toner image formed on the transferring and fixing roller is fused by heat, and then transferred onto the recording medium while being fixed. In this method, it is easy to heat the toner image on the transferring and fixing roller, and increase a width of a heating region to the toner image. Therefore, it is possible to sufficiently fuse a toner constituting the toner image until the toner image reaches a transferring and fixing nip area, even though the toner image is heated at a comparatively low temperature. In addition, the toner image reaches a certain level of a fused state by heat before introduced to a pressure contact area (the transfer fixing nip area) between the transfer fixing roller and the pressure roller. Accordingly, it is not necessary to increase a width of the transferring and fixing nip area. As a result, it becomes possible to decrease its power consumption required for a transferring and fixing, thus providing an advantage of speeding up of an image forming speed. Therefore, there are disclosed various kinds of proposals in order to further improve the concurrently transferring and fixing method.
For example, there is proposed an image forming apparatus including a toner image forming section, an intermediate transfer belt, a permeation belt, and a pressure roller. The toner image forming section includes a photoreceptor. The intermediate transfer belt is an endless belt member which is suspended in a tensioned state by two or more roller members, thus forms a moving path having a looped shape, and rotationally drives. The permeation belt is a transferring and fixing member having a shape of an endless belt which is suspended in a tensioned state by two or more roller members including a heating roller, thus forms a moving path having a looped shape, and rotationally drives, the transferring and fixing member forming a transfer nip area partially in pressure contact with the intermediate transfer belt. The pressure roller is a roller member in pressure contact, via the permeation belt, with one of the two or more roller members including the heating roller which suspends the permeation belt in a tensioned state, and thus forms the transferring and fixing nip area (refer to Japanese Unexamined Patent Publication JP-A 2000-194205, for example). According to the above-described image forming apparatus, in the toner image forming section, a toner image formed on the photoreceptor is transferred onto the intermediate transfer belt, and in the transfer nip area, the toner image on the intermediate transfer belt is transferred onto the permeation belt having a surface to which releasing agents such as a silicone oil are applied, and having a surface temperature of the transfer nip area controlled so as to be not lower than a softening temperature of the toner, and the toner image on the permeation belt is fused, and then in the transferring and fixing nip area, the toner image in a fused state is transferred and fixed onto the recording medium, to form an image on the recording medium.
As described above, when a belt member is used as a transferring and fixing member, it is necessary to increase a surface area of the heating roller in order to sufficiently heat the toner image on the transferring and fixing member. However, when the surface area of the heating roller is increased, an amount of heat discharge from the surface of the heating roller is also increased to decrease a heat efficiency, thus increasing its power consumption and making a configuration of the apparatus more complicated and larger. In addition, a wrapping distance of the belt member around the heating roller is decreased. Further, the toner image is heated via the belt member. Therefore, it becomes necessary to increase a heating temperature even higher than usual in order to sufficiently fuse the toner image. Also from this point of view, it is inevitable that its power consumption is increased. In addition, it may possibly become difficult to stably control a temperature of the belt member, causing a fixing failure. Moreover, when a releasing agent such as a silicone oil is applied on a surface of the belt member, the releasing agent is attached to the photoreceptor or the like via the intermediate transfer belt, causing an image failure. Further, the image forming apparatus disclosed in JP-A 2000-194205, has a configuration of heat circulation in which the intermediate transfer belt that has passed through the transfer nip area is cooled, and heat collected from the intermediate transfer belt is returned to the intermediate transfer belt in a region in front of the transfer nip area. This configuration cannot prevent a temperature of the intermediate transfer belt from gradually increase. Finally, the temperature thereof reaches around a temperature at which the photoreceptor or the like has heat deterioration.
Furthermore, there is proposed an image forming apparatus comprising a transferring and fixing section including an adhesive transferring and fixing belt having an endless belt shape which is suspended in a tensioned state by a heating roller and the other two roller members, and thus forms a moving path having a looped shape, and a pressure roller which is in pressure contact with the heating roller via the adhesive transferring and fixing belt, and thus forms a transferring and fixing nip area (refer to JP-A 2002-258630, for example). In the image forming apparatus, a toner image formed on a surface of a photoreceptor is electrostatically transferred onto an intermediate transfer belt, and the toner image on the intermediate transfer belt is transferred once again onto the adhesive transferring and fixing belt, and is introduced to the transferring and fixing nip area, and is superimposed on the recording medium to be heated and pressed, to thereby transfer and fix the toner image onto the recording medium. The image forming apparatus disclosed in JP-A 2002-258630 has the same disadvantage as in the image forming apparatus disclosed in JP-A 2000-194205 in that the belt member is used as the transferring and fixing member. In addition, the one disclosed in JP-A 2002-258630 has a configuration in which a silicone rubber layer is formed on the most surface of the belt member. Accordingly, a silicone oil that bleeds out from the silicone rubber layer is attached to the photoreceptor and the like via the intermediate transfer belt, causing an image failure.
Further, there is proposed an image forming apparatus comprising a toner image forming section; a intermediate transfer belt; a transferring and fixing section including a transferring and fixing roller having a heating section therein, and a pressure roller which is in pressure contact with a transferring and fixing roller and thus forms a transferring and fixing nip area; and a recording media heating section including a heating roller, and a pressure roller which is in pressure contact with the heating roller and thus forms a heating nip area for heating the recording medium, wherein the recording media heating section is provided upstream of the transferring and fixing section in a transporting direction of the recording medium (refer to JP-A 2004-151626, for example). This image forming apparatus, which uses roller members as a transferring and fixing member, has two heat sources corresponding to the transferring and fixing roller and the heating roller, thus making the configuration complicated and its power consumption increased. In addition, the heating roller is disposed adjacent to the intermediate transfer belt, thereby increasing a temperature of the intermediate transfer belt more than necessary. Accordingly, a temperature of a photoreceptor and the like may be possibly increased, thus causing deterioration of the photoreceptor, an image failure, or the like. Furthermore, there is no particular description regarding a transfer of the toner image from the intermediate transfer belt to the transferring and fixing roller, and it is thus understood that the toner image is transferred only by heat. However, when the toner image is transferred only by heat, a transfer failure may possibly occur.